Why has my deck frame rotted at the posts, and can I repair it?

Why deck posts rot at the ground line and not elsewhere, how to judge whether a post is repairable, and propping and replacing one without dismantling the deck.

Difficulty: advanced
Time: 8 hr
Read: 5 min
Safety: warning

Short answer

Posts rot in the 100mm either side of ground level, where there is both moisture and oxygen. Probe every post at that line. A post soft at ground level is replaced, not repaired: prop the bearer, cut the post out, and set the new one on a post base that keeps the timber clear of the ground entirely.

Timber buried in the ground does not rot uniformly. It rots in a narrow band at the ground line, because that is the one place with plenty of water and plenty of air at the same time. A post can be perfectly sound 300mm down and 300mm up, and hollow where it matters. That is why decks fail suddenly, and why the probe test at the ground line is the whole inspection.

Safety

A rotten post on a raised deck is a collapse risk and the deck should be out of use until it is repaired. Never work under a deck that is supported by the member you are cutting out — prop the bearer independently onto firm ground first, and keep out from under the deck while the prop is taking load.

What you'll need

Adjustable props or stout timber and wedges
Replacement post, treated for ground contact
Galvanised post base or post anchor
Concrete, or a bagged post mix
Screwdriver or a bradawl for probing
Reciprocating saw or a handsaw
Spirit level
End-grain preservative

Step by step

Probe every post at ground level, all round.Push a screwdriver hard into the post at ground line on all four faces, and 100mm above and below. Sound timber resists. If the blade sinks in, that post has lost its section where the load is highest.
Check the bearer and the joist ends above too.Water that rotted a post has usually also been sitting on the bearer top and in the joist hanger. Probe those before deciding the job is one post.
Work out what is holding the deck up while you work.Prop the bearer either side of the post you are removing, onto a slab or a solid pad on firm ground, and take the weight before cutting anything. On a large deck use two props per post.
Cut the old post out.Cut it above the rot and remove the top section, then dig out or break out the base. If it was set in concrete, breaking the collar out is usually easier than digging round it — and it must come out, because a new post cannot be founded in a broken-up old one.
Decide how the new post meets the ground.The best answer is not to bury it at all: a galvanised post base bolted or cast into a concrete pad, holding the timber 50 to 75mm clear of the surface with a drainage gap under it. A post that never touches soil never rots at the ground line.
Found the pad properly.Dig below any soft or made ground to firm bearing, and cast a concrete pad sized for the load with the post anchor set into it while the concrete is workable, checked plumb in both directions.
Fit the new post to the bearer with a proper connection.A post cap, an angle bracket or a housed and bolted joint. A post simply butted under a bearer relies on friction and does nothing to resist sideways movement.
Brace while you are in there.Diagonal bracing between posts in both directions is what stops a raised deck racking. If it was missing before, add it now — access will never be better.
Treat every cut and let the concrete cure before releasing the props.End-grain preservative on all cuts, and leave the props until the concrete has developed real strength. A day is not enough under load.

Tips

Joist and bearer tape — a self-adhesive bituminous strip run along the top of every joist and bearer — is inexpensive and adds many years to a deck frame by keeping water out of the screw holes and off the end grain.
If the deck is low and the ground under it is bare soil, membrane and 50mm of gravel under the whole footprint changes the humidity under there significantly.
Replace one post at a time. Propping and removing several at once turns a repair into a rebuild in the middle of the job.
Ground screws are worth pricing as a replacement footing. They install quickly, need no concrete and hold the timber clear of the ground.

Common mistakes

Concreting a post in with the concrete finishing level or dished — It forms a collar that holds water against the timber for days after rain. If a post is set in concrete at all, the collar must be domed to shed water away from the post.
Cutting out the rotten section and bolting a new piece on at ground level — The splice is in the one place with the highest bending moment and the wettest conditions. Posts are replaced full length, not spliced at the ground.
Standing the new post on the old concrete — The old base is cracked and its bearing is unknown. Break it out and cast a new pad.
Using general-purpose treated timber for a buried post — Treatment is graded by use class, and ground contact is a higher class than above-ground exterior. Ordinary treated timber in the ground has a short life.

If it doesn't work

Several posts are soft at ground line

Cause: The original construction buried untreated or lightly treated posts, or the ground under the deck holds water. — Fix: This is a frame rebuild rather than a repair. Price replacing all the posts on new pads with anchors, and deal with the drainage under the deck at the same time.

The post is sound but the bearer above it is soft

Cause: Water sitting on the horizontal top face of the bearer, often where a joist hanger holds debris. — Fix: Cut out and splice the bearer, or replace it. Then cap the bearer top with a strip of DPC or a proprietary joist tape so water cannot sit on end grain and in fixing holes.

A post is sound but moves

Cause: The connection to the bearer or the footing has failed, or the post was never braced. — Fix: Refix with a proper post cap or anchor and add diagonal bracing. Movement without rot is a connection problem.

Questions people ask

Can I just add a new post next to the rotten one?

It is a legitimate temporary measure to make a deck safe while you plan the repair, provided the new post is properly founded and connected. It is not a permanent fix, because the rotten post is still there decaying and its footing is doing nothing.

How long should a deck post last?

A ground-contact-treated post set in a domed concrete collar with good drainage will typically go fifteen to twenty-five years. A post held clear of the ground in a galvanised anchor is limited by the timber's exposure rather than by ground contact, and can outlast the deck boards several times over.

Is a concrete or steel post an option?

Yes. Galvanised steel posts and post systems are available for decking and they remove the problem entirely, at higher cost and with a different appearance. Concrete spurs — a concrete base with the timber post bolted above ground — are the traditional version of the same idea.